The present invention relates to a method for controlling the printing position of a video printer, whereby the initial position of printing paper is precisely controlled to prevent misplacement of the printing paper and thus deterioration of the picture quality.
Generally, the video printer employs a ribbon having three dyestuffs of yellow (Y), magenta (M) and cyan (C) which are sequentially sublimated by the heat of a thermal head and deposited on a single printing paper which is to circulate three times so as to print a video signal thereon with full colors. In this case, the amount of the generated heat by the thermal head is precisely controlled so as to regulate the amount of each of the dyestuffs subliminated, thereby freely making intermediate colors (color gradation).
Such conventional video printer as is shown in FIG. 1A includes a thermal head 100, a color ribbon 102, and a paten drum 106. The color ribbon 102 includes three colors of Y, M, C, which are bounded by a black strip with adjacent three colors, as is shown in FIG. 1B.
A printing paper 104 in close contact with the platen drum 106 runs between the platen drum 106 and the color ribbon 102 beneath the thermal head 100 at the same speed as the platen drum 106. The thermal head 100 heat generates according to the input video signals to sublimate each of the dyestuffs on the color ribbon 102, so that the sublimated dyestuffs are deposited on the printing paper 104, thereby reproducing the original colors of the input video signals. In order to fully reproduce the colors of the input video signal, whenever the printing paper 104 passes the theraml head 100, the position of the printing paper relative to the thermal head must always be the same, so that the dyestuffs of Y, M, C are sequentially deposited on the same spot with respect to each picture element. Here, the platen drum 106 circulates three times (one i.e., time for each of the three components Y, M, C) in order to complete one picture sheet.
Therefore, it is very important to precisely adjust the initial printing position of the thermal head 100, and if the initial printing position is misplaced, the printing spot with respect to each picture element is misplaced so as to separate the three color elements from each other, thus causing deterioration of the picture quality and forming a strip of M or C color along both sides of the printed picture as is shown in FIG. 3.
In such a conventional video printer, a circuit for sensing and controlling the position of the printing paper is constructed as shown in FIG. 2. The signal that is produced by photo-coupler 108 to sense the entrance of the printing paper 104 is amplified by amplifier 112, and is applied to microcomputer 114 to control the speed of the platen drum 106 according to the signal. For example, in order to reduce the printing time of the video signal, the platen drum is rotated at the normal speed when printing, while rotating at two times the normal speed when not printing.
If the time that is taken from the moment of the photo-coupler sensing the printing paper for the moment of the signal processing of amplifier 112 and microcomputer 114 to the normal speed of the motor 120 is different for each of the Y, M, C components, or the rotation of the motor 120 is affected by the backlash of gear 122, etc., the initial printing position is misplaced.
Thus, the speed control error of the motor 120 results when the signal sensed by the photo-coupler 108 is applied to the microcomputer 114, such that there occur differences of the sensing response time according to the moment when the program counter of the microcomputer 114 makes the sensing determination (present counting).
Consequently, if the initial printing position is misplaced, the printing position of the Y, M, C comonents is misplaced on the printing paper by a gap "a" or "b" as shown in FIG. 3, so that the picture quality is deteriorated throughout.